1. Field of the Invention
This invention relates to improved methods of monochromatic and color printing on various types of substrate surfaces. More specifically, this invention relates to methods of thermal transfer printing (e.g., thermal wax transfer, dye diffusion transfer or the like) images or information on a substrate such as paper.
2. Description of the Prior Art
Thermal transfer printing, as defined herein, includes the known printing methods of thermal wax transfer printing, thermal dye diffusion printing, and the like.
Thermal wax transfer printing involves the transfer of a colorant (e.g., magenta, cyan, yellow and, possibly, black), which is dispersed in a wax base material, from a carrier onto a substrate surface such as smooth paper in a controlled manner to generate an image. A thermal transfer print engine having a large number of independently activatable heating elements per unit of length is one apparatus that has been employed for this purpose. The carrier is most often placed within the print engine such that one side of the carrier is adjacent to the heating elements and a second, opposite side bearing the wax base material is positioned adjacent to the substrate surface upon which printing is intended.
To accomplish image generation, the print engine and substrate are moved relative to each other. If a color is intended to be deposited at a location on the substrate surface with which the printhead is aligned, the appropriate heating element is activated, and the carrier is locally heated to a temperature at or above the melting point of the colorant. When this happens, an amount of the wax-based ink in the colorant softens and adheres to the substrate at the desired location, breaking away from the carrier and the unheated or insufficiently heated colorant. In generating a subtractive color image, three (cyan, magenta and yellow) or four (cyan, magenta, yellow and black) sequential passes are made over the same substrate with different carriers, each of which is dedicated to one of the primary colors.
Dye diffusion printing involves the transfer of a dye colorant from a carrier, such as a polymer ribbon, onto a specialized substrate surface, such as a polyester sheet, in a controlled manner to generate an image. A thermal transfer print engine and three different color dye/carrier structures may also be employed in this type of image generation, utilizing similar heat-induced, subtractive color printing techniques. The operating principles for dye diffusion are different from those of thermal wax transfer printing, however. In dye diffusion applications, the amount of dye deposited at a single location can be varied, so that more subtle color gradations are achievable. Images of near photo quality have been produced using dye diffusion technology.
If a color is to be deposited at a location on the substrate surface with which the printhead of the thermal transfer print engine is aligned, the appropriate heating element is activated, and the dye/carrier structure is heated to a temperature sufficient to cause migration of an appropriate amount of dye, thereby releasing the dye from the carrier in the vicinity of the specially structured substrate. In this manner, the appropriate amount of dye contacts and penetrates the substrate through molecular dispersion of the dye in the substrate at the desired location.
Many types of paper, particularly the bond-type paper which is popular in the United States, exhibits a rough surface, featuring plateaus and voids. Conventional prior art thermal transfer printing techniques cannot be effectively used with such paper, because the voids in the paper substrate surface structure cause adherence problems or broken ink or dye dots.
Also, most paper is not chemically compatible with diffusion dyes in a manner to provide a solid-in-solid dye-substrate solution. As a result, bright colored images cannot be generated on such substrates using dye diffusion technology. Since "plain paper" exhibiting a rough surface is less costly than specialized substrate materials, and high quality images are obtainable using thermal transfer techniques, methods of color printing on plain paper substrates are desirable.
One method of expanding the range of substrates useful in thermal transfer printing involves the choice of inks or dyes used to generate the image. For example, Abe and Kitamura suggest the use of inks having increased tensile strength to decrease the number and severity of image voids for monochrome printing applications in an article entitled "Relation Between Dynamic Characteristics of Thermo-Fusible Ink and Print Quality in Thermal Transfer Printing," Journal of Imaging Technology, Vol. 17, No. 3, pp. 119-122, June/July 1991.
Another method of increasing the number of usable substrates proposed for use in monochrome printing applications is the employment of a carrier with multiple layers of ink disposed thereon. Theoretically, the first ink layer deposited on the substrate surface improves the deposition of the simultaneously deposited second layer. U.S. Pat. No. 4,623,580 issued to Koshizuka et al. is directed to this technology.
Such monochrome technology is not amenable to the multi-pass methodology employed in subtractive color printing, however. In multi-pass color printing, the deposition of multiple layers of colorant can result in loss of image resolution and adhesion as a consequence of the cumulative thickness of the deposited layers.
More recently, it has been suggested to deposit a precoat onto the substrate surface which is smoother and more chemically compatible with the intended colorant than the substrate surface is. U.S. Pat. Nos. 4,527,171 issued to Takanashi et al., 4,670,307 issued to Onishi et al. and 4,704,615 issued to Tanaka are examples of thermal transfer printing systems which utilize a fusible binder material that is deposited as a precoating onto the substrate prior to printing.
Takanashi et al. coat the entire substrate upon which printing is intended with a layer of the binder material. This method effectively prepares the substrate for deposition of colorant during printing, but consumes a large amount of the binder which is used during the precoating process. In addition, it results in a discernable smoothening of the surface of the substrate, which may be objectionable to those who prefer the look and feel of plain paper.
In Tanaka, the precoating is applied to the substrate on a pixel by pixel basis, to the same pixel locations as where the deposition of colorant is intended. The precoating is applied to a slightly broader area at each pixel location than the colorant, as a result of a longer pulse width being applied to the printhead element during deposition of the precoat material. While the Tanaka system preserves the look and feel of plain paper for that portion of the substrate which had not been printed on, certain problems still exist. Specifically, it has been found that, in some instances, it is difficult to effectively apply the precoating to the substrate in discrete one pixel intervals. Such problems are most common in areas where the desired image requires that the colorant be applied only very sparsely to the substrate. In such areas, it appears that the combined effect of heat loss from a single activated heating element on the printhead and the transitional period needed to bring the heating element to its operational temperature result, at times, in a less than satisfactory deposition of the precoat material to the substrate.
The precoat material in Tanaka was designed so as to fill the voids or valleys-which are defined in the surface of a rough substrate, rather than bridging over such voids or valleys. This is typical of the precoat technology which is in common use today. Japanese patent publication 62-77987, assigned to Mitsubishi Chemical Industries, Ltd., discloses the deposition of a combined stratum of an adhesive layer and an ink layer to a rough substrate in such a manner that the adhesive layer bridges the voids or valleys in the substrate, but the deposition of a bridging-type precoat or surface preparation layer which is applied separately from the colorant has not heretofore been known to those skilled in this area of technology.
It is clear that there has existed a long and unfilled need in the prior art for an improved system and method for making printed products which overcomes the disadvantages discussed above.